1. Field of the Invention
The present invention relates to a communication apparatus and a communication method, a communication system, and a computer program in which a communication is mutually performed among a plurality of radio stations such as a wireless LAN (Local Area Network) or a PAN (Personal Area Network), in particular, a communication apparatus and a communication method, a communication system, and a computer program in which a plurality of communication stations operating in an autonomous distributed manner are connected in Peer to Peer.
More specifically, the present invention relates to a communication apparatus and a communication method, a communication system, and a computer program of a Peer to Peer style in which the communication stations operating in an autonomous distributed manner mutually exchange beacon frames to operate a communication, in particular, a communication apparatus and a communication method, a communication system, and a computer program of the Peer to Peer style in which beacon timing information received from an adjacent station is placed in the beacon frame to mitigate a beacon problem caused by a hidden terminal.
2. Description of the Related Art
As a system of liberating wires used in a wired communication system in a related art, a wireless network draws attention. For example, IEEE (The Institute of Electrical and Electronics Engineers) 802.11a, IEEE802.11b, or IEEE802.1g is typical as a wireless LAN standard. With the wireless LAN, a flexible internet connection can be established. Not only the existing wired LAN is replaced, but also internet connection means can be provided in public places such as a hotel, an airport lounge, a station, and a cafe. The wireless LAN has been already widely spread, and a wireless LAN function is now being common to be mounted not only to an information device such as a personal computer (PC) but also to a consumer electronics (CE) device such as a digital camera or a music player.
In order to construct a LAN by using a wireless technology, a method is generally used of providing one apparatus functioning as a control station called “access point (AP)” or “coordinator” in an area and forming a network under an overall control of this control station. The control station performs a synchronized wireless communication in which access timings of a plurality of terminal stations existing in the network are adjusted, and the respective terminal stations are mutually operated in synchronization.
In addition, as another method of constructing the wireless network, an “ad-hoc communication” is devised in which all the terminal stations are on an equal footing and operated in Peer to Peer in an autonomous distributed manner, and the terminal stations decide the access timings by themselves. In particular, for a small-sized wireless network constructed by a relatively small number of clients located close to each other, the ad-hoc communication is regarded as an appropriate system in which arbitrary terminals mutually can directly perform an asynchronous wireless communication without utilizing a particular control station.
For example, the networking in IEEE802.11 is based on a concept of BSS (Basic Service Set). The BSS is structured by two types including BSS defined by an “infrastructure mode” where the control station exists and IBSS (Independent BSS) defined by an “ad-hoc mode” which is structured by only a plurality of MTs (Mobile Terminal: mobile station or terminal station).
Furthermore, other than the ad-hoc network regulated by IEEE802.11, a communication system is developed in which the respective communication stations operating in an autonomous distributed manner are connected in Peer to Peer. For example, a “multi-hop communication” in which a plurality of communication stations relay frames solves a problem that all the communication parties are not particularly accommodated in a range where the radio waves reach. With the “multi-hop communication” in which the plurality of communication stations relay the frames, it is possible to mutually connect a large number of communication stations. Currently, as a task group (TG) in IEEE802.11, a standardization of the multi-hop communication is in progress. In the present specification, the wireless network carrying out the multi-hop communication is referred to as “mesh network”, and the respective communication stations structuring the mesh network are referred to as “mesh point (MP)”.
For example, a wireless communication system is proposed in which a network is structured while the respective communication stations transmit beacons in which information related to the network is described to each other, and a sophisticated determination is performed regarding a communication state at another communication station on the basis of the beacon (for example, see WO2004/071022). By using a similar method, the mesh network can be structured.
FIG. 15 illustrates a communication sequence example in which a wireless communication system in which a communication is performed in an autonomous distributed manner while the respective communication stations exchange beacon signals. In the example shown in this drawing, as communication stations participating the network, two stations STA1 and STA2 exist in a mutually communicable range. The respective communication stations set respective TBTTs (Target Beacon Transmission Time) and periodically transmit beacon signals. Then, as the respective communication stations extract information of the adjacent MT, as occasion demands, the communication station periodically receives the beacon signals from the other communication station. It should be noted that beacon transmission cycles are not regularly the same for all the communication stations, but herein, for simplicity of the description, the respective communication stations are supposed to transmit the beacons in the same beacon transmission cycle.
Here, in a Peer to Peer communication system, a hidden terminal problem is generated in general. In a case where a communication is performed between particular communication stations, a hidden terminal refers to a communication station which can hear from one communication station functioning as a communication party, but the communication station is difficult to hear from the other communication station. Negotiation is difficult for the hidden terminals to perform, and transmission operations may collide with each other. With reference to FIGS. 16 to 18, the beacon hidden terminal problem and a coping process thereof in the Peer to Peer communication system where the communication is carried out while the communication stations operating in an autonomous distributed manner exchange the beacon signals will be examined.
FIGS. 16A to 16C exemplify a situation where the beacon hidden terminal problem is generated in the Peer to Peer communication system.
In a situation shown in the left side of FIG. 16A, only one communication station STA0 exists. The right side of FIG. 16A shows a situation where the STA0 transmits the beacon at every 100 milliseconds. In the same drawing, B0 denotes a beacon signal transmitted by the STA0 in a cycle of 100 milliseconds.
Subsequently, the left side of FIG. 16B shows a situation where a new communication station STA1 appears in a communication range of the STA0. In this case, as the STA1 can directly receive the beacon B0 of the STA0, the STA1 appropriately find a timing during which a collision with this beacon is not generated and sets the TBTT of its own station. The right side of FIG. 16B shows a situation that as a beacon transmission time B1 of the STA1, just around a middle part of the beacon transmission cycle at 10 milliseconds of the STA0 is selected.
Furthermore, subsequently, the left side of FIG. 16B shows a situation where a STA2 (that is, which functions as a hidden terminal for the STA0 appears at a location which is within the communication range of the STA1 but no direct radio waves do not reach from the STA0. In this case, as the STA2 can directly receive the beacon B1 from the STA1, it is possible to select a time at which the collision with this beacon is not generated for the beacon transmission time B2 of its own station. However, the STA2 does not know the existence of the beacon B0 transmitted from the STA0 functioning as the hidden terminal, and therefore the beacon may be transmitted at the same time as the STA0. In such a case, in the STA1, the beacons of the STA2 and the STA0 are collided with each other, which leads to a significant problem for the operation of the network.
For example, such a method is proposed that the respective communication stations place the time information of the beacon received from an adjacent station on a beacon signal to notify each other (for example, see WO2004/071022). According to this method, the respective communication stations analyze reception beacon time information described on the beacon received from the adjacent station to detect the beacon transmission time of the hidden terminal, so that it is possible to avoid the collision.
FIG. 17 shows a format example of a beacon frame. Although omitted in the same drawing, in general, the beacon frame also includes a field indicating in which cycle the communication station which has received the frame transmits the beacon (in the frame format, it should be understood that this is included as a part of Other Information).
Various formats are conceivable for the above-mentioned “time information of the reception beacon”. The frame format example shown in FIG. 17 can be represented by “Time Stamp value” which is a timer value for performing a time management of its own station and “beacon timing information (Beacon Timing)”. The respective communication stations activate a timer for counting up a reference time of its own apparatus in its own apparatus, and a timer value at a moment when the frame is transmitted is placed as “Time Stamp” value in the beacon frame. Also, FIG. 18 shows a content of “beacon timing information”. In the beacon timing information shown in the drawing, an element ID is used as a header, length information of the element follows, and after that, respective pieces of information “STA ID value”, “beacon reception time”, and “beacon interval” are placed for the number of reception beacons. For example, the communication station receiving beacons from the two communication station STA0 and STA1 describes three information sets with respect to each of the STA0 and the STA1. Herein, “STA ID value” is a value for identifying a transmission source of the beacon. “Beacon reception time” is information indicating a reception time of the beacon, and a timer value of its own apparatus when the beacon is received is described. “Beacon interval” is a value indicating a transmission frequency of the beacon (a transmission cycle of the beacon frame at the transmission source of the beacon).
By placing the beacon timing information in the beacon frame, the beacon hidden terminal problem in the Peer to Peer communication system can be significantly mitigated. In the example shown in FIG. 16C, the STA2 receives the beacon from the STA1 to analyze the beacon timing information, so that it is possible to know “at what time in further the STA1 will receive the beacon from the STA0 (hidden terminal)”. Thus, it is possible to set the beacon transmission time of its own station at a time when the hidden terminal beacon is not overlapped.
However, the solving method for the hidden terminal problem of placing the beacon timing information in the beacon frame invites several new technical problems.
The first problem corresponds to a situation that in the format of the “beacon timing information” shown in FIG. 17 or 18, if the beacon time information from a large number of peripheral communication station is placed, the information amount to be placed on the beacon is large, and the beacon frame becomes enormous.
For example, when “STA ID value” is represented by 1 octet, “beacon reception time” is represented by 2 octets, and “beacon interval” is represented by 2 octets, respectively, the size of the beacon timing information for one communication station becomes 5 octets. In this case, if the beacon timing information for 16 peripheral communication stations is aimed to be placed, the total information amount of 80 octets (=(1+2+2)×16) is used, and the size is difficult to ignore.
The second problem corresponds to a situation that when the beacon transmission cycles for the respective communication station are varied, it is difficult to determine as to the presence or absence of the beacon collision. In the communication sequence example shown in FIG. 15, for simplicity of the description, the beacon transmission cycles of the communication stations participating the network are all the same. When the beacon transmission cycles of the respective communication stations are the same or in a relation of integral multiple, it is easy to determine whether or not the collision is continuously generated towards the future, but the network operation method is not limited to the above. For example, in a case where one communication station transmits the beacon at 200 millisecond interval, and the other communication station transmits the beacon at 300 millisecond interval, the beacon collision is intermittently generated, and a vague situation is generated. In this case, the determination is unclear as to regard this event as the collision or not.